The mouse in your car is probably getting out at night to find food and water, otherwise it would starve. Continue to place traps in your garage. You might also try traps in the car itself. I would not use poison, if the sick mouse goes back into the car to die, you could have a real problem getting rid of it when it starts to, well, rot.
If the have been eating something in your garage, like pet food, bait the trap with that if you can. Mice avoid new foods at first, so a familiar food may work better. Also, try and clean up your garage...get rid of food, next materials and hiding places, and seal up as many holes and gaps as you can find.